蒙古高原古近纪两次重大气候转折期哺乳动物牙齿釉质氧碳稳定同位素研究

During the Cenozoic era, two pronounced climate events happened and further influenced modern climatic and ecological system. One is the Paleocene-Eocene Thermal Maximum (PETM) when the global temperature rapidly warmed by 5-8°C. The other is the cooling even of Eocene-Oligocene transition which lower an average temperature of 4°C. Both evens made an abrupt faunal turnover.. The marine records of these transitions have been extensively studied. However, significantly less research has focused on continental climate change at the time. Comparing to many other proxies of paleoenvironmental reconstruction, stable isotope compositions of mammal bioapatite are widely used, because it can survive most diagenesis and preserve a primary isotopic signature for its low of organic compound content, low porosity and high compactness.. The δ13C value of herbivorous bioapatite is controlled by that of ingested plant and can therefore be used to mirror dietary strategies, ecological niches, vegetation cover, as well as precipitation. The δ18O values of bioapatite is controlled by that of ingested water and mediated by body temperature and physiology. As body water is a function of meteoric water ingested directly by drinking and indirectly through water in food and inspired air. The δ18O value of bioapatite can be used to infer drinking behavior and habitat partitioning, as well as air and surface water temperature.. Variability in time-series enamel isotope values recovered from incremental structures within a single tooth records a chronological record of dietary and environmental input throughout the period of enamel formation, which ranges from days to years in mammals. Following this approach, intra-tooth variability in δ13C and δ18O values from enamel can be used to address problems requiring seasonal resolution, including seasonality of diet, migration, as well as climate seasonality.. In this project, we plan to test the δ13C and δ18O variations from bulk samples of the mammal tooth enamel spanning P/E boundary in Erlian Basin and that of E/O boundary in Sanshenggong area, the age of each is strictly controlled by the Paleogene biostratigraphy and chronostratigraphic framework. Using the isotopic information we attempt to reconstruct palaeoenvironment in terms of mean annual precipitation, air and surface water temperatures. Besides, by studying the intra-annual oxygen stable isotopic variation from serial samples of single teeth spanning these two boundaries, which is controlled by the short-period time scale and tooth enamel formation and mature pattern given by the incremental structures of enamel, we seek to understand the patterns and variability of seasonality before and after these transitions. For the first time, by comparing the variability of palaeoenvironmental parameters spanning the two transitions, we can further discuss to what extent do mammal fauna turnover correspond with the climate changes in the fossil records.

古近纪两次最显著的全球性气候事件，一是古新世/始新世之交的高温事件，二是始新世/渐新世之交的降温事件，它们对全球气候和生态系统产生了巨大的影响，导致了陆相动物群的更替，使哺乳动物经历了由古老类群向现代类群的演变。在哺乳动物对环境的响应研究中，牙齿釉质氧碳同位素被认为是恢复古气候变化的有效指标。本项目旨在中国古近纪哺乳动物年代体系和相关地层学研究基础下，辅以哺乳动物牙齿釉质生长纹所提供的短期时间标尺和生长矿化模式的控制进行相关研究。研究剖面位于内蒙古二连盆地和三盛公地区，分别含有两次气候变化界线上、下的地层及大量哺乳动物化石。在已有和进一步对化石属种鉴定的基础上，开展牙齿釉质全样和微区氧碳稳定同位素研究，以期区分各类哺乳动物不同的食性和栖息生境，恢复蒙古高原两个界线上下的古气温、降水量、表水温度和季节性气候变化，并深入探讨东亚地区哺乳动物群演化、区域性气候变化和全球气候升、降温变化的对应性。

古近纪古新世/始新世之交的高温事件对全球气候和生态系统产生了巨大的影响，导致了陆相动物群的更替，使哺乳动物经历了由古老类群向现代类群的演变。在哺乳动物对环境的响应研究中，牙齿釉质氧碳同位素被认为是恢复古气候变化的有效指标。本项目对内蒙古二连盆地古新统/始新统界线上下，时间跨度约为十个百万年，三组十一个层位，5目8科8属9种，共计四百余件哺乳动物样品做了牙齿釉质结构碳的碳氧稳定同位素分析。结果显示，该时间层段的哺乳动物的食性和栖息环境呈现了极高的多样性和丰富性。碳稳定同位素的分布范围为-13.7~-6.7‰，食物类群全落在C3植物的范围之内。氧为-11.9~1.3‰，显示在该时间段内，动物群广布与各个生态位。通过对可追溯的、亲缘关系较近的同一支系类型的区分取样分析发现，各类群有不同的变化规律，如奇蹄类稳定碳同位素在古新统/始新统界线之后，出现一个明显的负漂，显示出了与PE全球升温事件的耦合性，但呈现了一定的滞后。恐角兽类的稳定碳同位素则呈现出逐步平稳下降的趋势，受温度急升环境剧变的影响较小，其取食更倾向于裸子植物类型，而其碳稳定同位素持续下降，更可能跟被子植物在古近纪早期的逐步扩散有关，因而最终导致了恐角兽类在渐新世的绝灭。氧同位素也被证实，能用作很好的参数，来分辨和鉴定出水生或半水生的哺乳动物类群。而在组织学方面，也选取了多种哺乳动物牙齿釉质做了切片研究。针对部分单颗的存在鉴定问题的牙齿，利用牙齿釉质微观结构对其进行了形态学分类研究。组织学上的进展，也给本项目的实施提供了更明确的分类上的依据，进而更精确了项目取样属种分类的确定性，并减少了属种差异对环境变化解释的影响。对部分大型哺乳动物臼齿和啮形类的不断生长的门齿的生长纹组织学研究，建立了可追索的釉质形成和成熟的模式，为原位的稳定同位素研究提供了良好的靶向图，并为微区氧碳稳定同位素方法研究季节变化研究提供了精确的短周期和季节变化的时间标尺。